Your flight to the major airline hub reaches cruising altitude without incident. While you consider checking the in-flight menu for one of those drinks that come with a little umbrella, you are interrupted by the kid sitting next to you.

Their handheld game console won't turn on! They ask if you can take a look.

Read the full puzzle.

using System;
using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2020.Day08;

record Stm(string op, int arg);

[ProblemName("Handheld Halting")]
class Solution : Solver {

    public object PartOne(string input) => Run(Parse(input)).acc;

    public object PartTwo(string input) =>
        Patches(Parse(input))
            .Select(Run)
            .First(res => res.terminated).acc;

    Stm[] Parse(string input) =>
        input.Split("\n")
            .Select(line => line.Split(" "))
            .Select(parts => new Stm(parts[0], int.Parse(parts[1])))
            .ToArray();

    IEnumerable<Stm[]> Patches(Stm[] program) =>
        Enumerable.Range(0, program.Length)
            .Where(line => program[line].op != "acc")
            .Select(lineToPatch =>
                program.Select((stm, line) =>
                    line != lineToPatch ? stm :
                    stm.op == "jmp" ? stm with { op = "nop" } :
                    stm.op == "nop" ? stm with { op = "jmp" } :
                    throw new Exception()
                ).ToArray()
            );

    (int acc, bool terminated) Run(Stm[] program) {
        var (ip, acc, seen) = (0, 0, new HashSet<int>());

        while (true) {
            if (ip >= program.Length) {
                return (acc, true);
            } else if (seen.Contains(ip)) {
                return (acc, false);
            } else {
                seen.Add(ip);
                var stm = program[ip];
                switch (stm.op) {
                    case "nop": ip++; break;
                    case "acc": ip++; acc += stm.arg; break;
                    case "jmp": ip += stm.arg; break;
                };
            }
        }
    }
}

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